We propose to create a minimized and modified streptokinase having improved properties relative to the known plasminogen activators currently used in thrombolytic therapy. New methods developed by KAIROS for protein minimization and high-throughput screening of enzyme variants wili be used to find smaller versions of the native protein that have maximal activity on fibrin-containing clots. The minimized streptokinase (mSK) will be designed to have: (1 ) increased specificity for fibrin; (2) increased resistance to degradation by plasmin and other proteases; (3) resistance to inhibitors; (4) thermal stability; and (5) low production cost. In addition, a minimized bacterial protein offers the potential benefit of reduced antigenicity. If successful, a minimized and optimized streptokinase will provide a new alternative biopharmaceutical for the thrombolytic therapy market, which is estimated to be worth over $200 million per year in sales. Successful modification of streptokinase will also provide a model for engineering other therapeutic proteins.